The present invention relates to an improvement in the sealing of leak-proof electrochemical cells, particularly concerning the sealing of the cell bottom. The invention relates to a method of achieving this closing, as well as to cells obtained by this method.
The leak-proof closing of electrochemical cells poses difficult problems, mostly due to corrosion and the formation of gaseous pressure inside the cell while it is working. Generally speaking, the positive electrode, electrolyte and depolarizer mass are enclosed in a metallic cup, most often of zinc, magnesium, or some other metal which fulfils the role of a negative electrode; as the bottom of such a cup constitutes the negative terminal of the cell, special precautions should be taken to establish a satisfactory electrical contact between this bottom and the exterior, at the same time ensuring that there is perfect tightness and that suitable mechanical solidity between the bottom and the exterior exists. To solve this problem, the method used consists in fixing a suitably sealed metallic part against the outside surface of the bottom of the negative electrode. The role of this metallic part is to ensure electric contact between the cup and the exterior, and at the same time to mechanically protect the bottom of the cell. The problem still existing is to establish the required tightness between the above-mentioned metallic part and the remainder of the cell. Plastic material is usually used for interposition and/or partial envelopment, but this problem, which appears to be simple, is in fact quite difficult to solve with acceptable economic means. The joint between the metallic part and the whole of the cell is the delicate point. To accomplish this joint, attempts have been made to hold the metallic part in place by a plastic casing which surrounds the cup of the cell. However, it is then necessary for this plastic casing to be compressed by an external metallic sleeve, as described in U.S. Pat. No. 2,802,042, to Anthony et al. As this system has not given satisfaction, the metallic part has been given a dish shape, the cylindrical side-walls of which are fitted onto constricted walls of the part of the cup adjacent to the bottom thereof. The constriction is such that the outer diameter of the dish so fitted is equal to the diameter of the remaining part of the cup, in such a way that there is no protrusion at dish level on the plastic casing which encloses the whole. Such an arrangement is described in U.S. Pat. No. 3,168,420 and 3,433,681 to Jammet. As such fitting onto the constrained part is not sufficient to ensure that the metallic dish be held tight and sealed, the patents to Jammet provided for compression of the whole by a metallic ring tightening the plastic casing. This ring, by means of the plastic casing, compresses the side-walls of the dish against the constricted parts of the cup. This system involves some rather exacting operations as far as the manufacture of the cell is concerned, because it is first of all necessary to grip the bottom of the cup, position the dish, cut the edges which protrude downwardly from the plastic casing, then fit the metallic ring onto the whole and, finally, to compress it into a mould to reduce its diameter in order to obtain the required compression. Manufacture therefore comprises 4 or 5 additional operations from the moment when the cup is charged with the components of the cell and covered with its plastic casing.
Another solution to this problem is described in U.S. Pat. No. 3,501,351 to Karobath. Instead of a dish, there is used a metallic part having a diameter equal to or smaller than the bottom of the negative electrode. This part has perforated apertures to allow the passage of a plastic material. To fix this part, first of all the bottom of the cup is lined with a plastic material, then the part is pressed against the bottom of the cell to embed it into the plastic material fixed to the bottom. The periphery of the plastic casing surrounding the cell is then bonded to the material which covers the part. Due to the apertures, the plastic material is well anchored to the metallic part, on both sides of the apertures, to improve the seal. However, the manufacture of cells by this method requires at least three additional operations, and fixing by embedding the metallic part is not altogether satisfactory.